Disproportionation of toluene can convert toluene into more valuable benzene and xylenes. The xylene products are generally mixtures of three isomers in thermodynamical equilibrium, and the most valuable p-xylene accounts for only about 24%. Therefore, shape selective disproportionation as a new process is proposed to selectively produce p-xylene. Conventional catalysts for the toluene disproportionation contain mordenite having a larger channel diameter as the main active component and therefore exhibit no shape selectivity to the three isomers of xylenes. ZSM-5 molecular sieve has a three dimensional channel system consisting of 10-member rings. It has been found that the channels of ZSM-5 molecular sieve permit quick diffusion of p-xylene having a molecular diameter of 0.63 nm, while o-xylene and m-xylene having a molecular diameter of 0.69 nm have much less diffusion coefficient. The diffusion coefficients of the species present in toluene disproportionation system in the channels of ZSM-5 molecular sieve have the following relationship: benzene≧toluene>ethylbenzene≅p-xylene>o-xylene≅m-xylene. This implies there is a possibility that toluene disproportionation may be shape selectively catalyzed to obtain a concentration of p-xylene isomer in the xylene product much higher than the thermodynamical equilibrium concentration of p-xylene. However, since the p-xylene-rich product diffused out from the molecular sieve channels will undergo isomerization reaction on acid sites existing on external surfaces of the molecular sieve, and the rate of the isomerization reaction is much higher than the rate of the disproportionation reaction, the product will quickly reach thermodynamical equilibrium composition. Although some investigations show that a shape selectivity can be achieved by using ZSM-5 having a large grain size at a higher space velocity and a lower conversion, such a process is not practical in industry.
U.S. Pat. Nos. 5,367,099, 5,607,888 and 5,476,823 disclose the modification of ZSM-5 molecular sieve structure, i.e., reducing the size of pore opens and shielding acid sites on external surfaces, to prepare a catalyst for selective toluene disproportionation. In the preparation of the catalyst, a thermally decomposable polymer is deposited on the external surfaces of the molecular sieve, and then the molecular sieve is subjected to a high temperature treatment to thermally decompose the polymer to form an inert coating, which will shield the acid sites on the external surfaces of the molecular sieve and also reduce the size of the pore opens to an extent. Such a surface modification enhances greatly the p-xylene selectivity of the catalyst.
U.S. Pat. No. 5,365,003 discloses a process and a catalyst for a shape selective hydrocarbon conversion. The catalyst is prepared by a process comprising: agglomerating a mixture comprising molecular sieve crystals, organosilicon compound and optionally a binder material; and calcining the resulting agglomerate. The catalyst may be subsequently contacted with a mixture of a high-efficiency p-xylene trim selectivating agent and substituted aromatic at reaction conditions for converting toluene to xylene to produce a twice selectivated catalyst, wherein the high-efficiency p-xylene trim selectivating agent includes, for example, organosilicon compounds.
Chinese Patent Application No. 00119772.X discloses a noble metal-modified catalyst for toluene selective disproponionation, comprising 20 to 90 wt % of ZSM-5 molecular sieve in hydrogen form, 0.005 to 5 wt % of at least one noble metal selected from ruthenium, rhodium, palladium, rhenium, platinum and gold, and 9 to 75 wt % of silica or alumina as a binder. The catalyst further comprises optionally at least one element selected from chromium, nickel, molybdenum, tungsten, antimony and bismuth. Although the catalyst containing noble metal-modified ZSM-5 molecular sieve in hydrogen form may enhance catalytic activity for toluene selective disproportionation, the noble metal-modified molecular sieve catalyst will cause relatively significant toluene hydrogenation and dealkylation side-reactions, thereby reducing the yield of the p-xylene product.
Although some investigations have been conducted in the shape selective catalysis of toluene disproportionation, there still need catalysts for toluene shape selective disproportionation, which have higher catalytic activities and, at the same time, will cause little or no toluene hydrogenation and dealkylation side-reactions.